1. Field of the Invention
This invention relates to a method for coating particulate material with a thermosetting resin and the product produced by such method.
2. Background of the Invention
Resin-coated particles have been proposed for use in a wide variety of operations including molding (resin-coated sand), reinforcing of concrete or cement (resin-coated steel fibers), and oil field hydraulic fracturing (resin-coated propping agent). The present invention will be described mainly in connection with resin-coated particles employed as a propping agent in hydraulic fracturing operations; it should be understood, however, that the particles coated in accordance with the method of the present invention can be used in the operations mentioned above as well as other operations which employ free-flowing, resin-coated particulate material.
Hydraulic fracturing is a technique for stimulating the production of subterranean formations. The technique normally involves (1) injecting a viscous liquid through a well and into the formation at a sufficient rate and pressure to overcome the earth stresses and form a crack (fracture) in the formation; and (2) placing a particulate material (propping agent) in the formation to maintain the fracture in a propped condition when the injection pressure is released. The propped fracture thus provides a highly conductive channel in the formation. The degree of stimulation afforded by the hydraulic fracture treatment is largely dependent upon the permeability and the width of the propped fracture.
Although a variety of particulate materials have been proposed for use in hydraulic fracturing, silica sand remains the most common propping agent currently used. Sand, however, is not an ideal propping agent. Its relatively low compression strength and tendency to fragment greatly restrict its effectiveness at closure stresses above about 5000 psi. At higher stress levels, the fines produced by particle disintegration plug the interstices of the propped fracture, drastically reducing fracture permeability.
Efforts to develop a propping agent which exhibited improved strength and less tendency to disintegrate led to the employment of resin-coated particles. Tests have shown that permeability retention of these particles is far better than sand under the same conditions. However, techniques for coating the particles were either unsuited for preferred materials or were too expensive, with the result that resin-coated particles have not been used extensively as a proppant. The main problem associated with the coating process is that of particle agglomeration.
In one coating technique, a liquid resin is applied to a batch of particles. The coated particles are then agitated in a suitable oil bath until the resin cures. The agitation tends to prevent the particles from sticking together (agglomerating). Tests have shown, however, this technique does not work for all resins. With epoxy, for example, the particles usually cure into one large agglomerated mass. If sufficient agitation is provided to prevent agglomeration, this technique produces irregular surface coatings on the particles. These irregularities are believed to be due to the manner in which the resin-coated particles are cured. Under agitation, the coated particles collide and temporarily bond together at contact areas. Agitation however breaks the bond permitting each particle to repeatedly collide with and break away from other particles. As the resin reaches a tacky condition, the forces tending to separate two particles pull or stretch the bond holding particles together until the bond breaks. The stretched resin in the partially cured state retains its extended form leaving an irregular projection or spike on the coating surface. During the curing step, collisions and bond disruptions are constantly occurring on each particle with the result that each particle has several irregular projections. These projections are undesirable mainly because they produce thin coatings on the particle between the projections and thereby reduce the mechanical strength of the particle. Also, there is increased tendency of such particles to bridge an opening which could lead to "sand out" in hydraulic fracturing.